Naturally occurring radioactive material (hereinafter "NORM") is present in varying concentrations in groundwater and the like, in water supply wells, oil production wells, gas production wells, and as byproducts in mining operations. In the oil field, NORM is the result of material that has been extracted from the producing zone and is deposited on the equipment in the form of solids, films, pipe scale, sediment, and the like. The radioactive material is typically radium 226, radium 228, radon 222, thorium 232, uranium 235, uranium 238, lead 210, polonium 210, and other naturally occurring radionuclides. Typically these radionuclides are .alpha., .beta. and often .gamma. emitters which have a long half life. Such radionuclides are believed to be associated with toxic and carcinogenic effects. Strict health-based limits thereon have been enacted or are under consideration.
For example, the process equipment used in various petrochemical plants, refineries, and the like, and associated piping is exposed to high levels of NORM. The disposal of equipment having a high level of NORM has come under increased scrutiny, particularly in oil-producing states such as Louisiana and Texas. Thus, many companies are stockpiling equipment which will need to either be cleaned for reuse or decontaminated for disposal. Thus, currently the most common practice other than stockpiling is to ship the equipment to a radioactive waste facility which have their own environmental problems. There are also several costly mechanical methods on the market for removing NORM. These include ice, sponge, or carbon dioxide blasting. These methods have limitations in that these methods are more applicable to pipe scale and other solid forms of NORM as compared to NORM deposited in solution or as a film which adheres to metal surfaces and is difficult to remove.
Methods are known to remove radioactive materials from surfaces such as those found in nuclear reactors. For example, U.S. Pat. No. 4,537,666 to Murray et al. describes the typical system as treating the surfaces with an oxidizing solution, such as one containing an alkaline permanganate. This is followed by treatment with a decontamination solution which is an aqueous solution of a chelate, such as ethylenediaminetetraacetic acid (EDTA), and a solubilizing agent, such as a mixture of oxalic acid and citric acid. The chelate forms a complex with the metal ions from the deposits and solubilizes them, and, thus prevents them from precipitating out of the solution at another location in the cooling system. The decontamination solution is circulated between the cooling system and a cation exchange resin. The chelated metal ions are deposited on the cation exchange resin, freeing the chelate to solubilize additional metal ions in the deposit.
The difficulty with this decontamination process, according to Murray et al., is that both the chelates and the cation exchange resin complete for the metal ions. As a result, the metal ions do not readily leave the chelate and attach themselves to the ion exchange column. This means that long resin contact times are required, and that the ion exchange column effluent may contain relatively high metal ion concentrations. Murray et al. proposes to remove the metal ions by passing the decontamination solution through a porous DC electrode.
Other exemplary methods for removing nonnaturally occurring radioactive materials are proposed in U.S. Pat. Nos. 4,704,235 to Arvesen; 4,729,855 to Murray et al.; 4,792,385 to Snyder et al.; and 5,111,887 to Morris et al.
Despite the general availability of methods of removing naturally occurring and nonnaturally occurring radioactive materials, there continues to be a need for removing NORMs from surfaces exposed to the same, and particularly NORM deposited as a solution or film and adhered to surfaces.